The invention relates to circuitry that permits rapid "power up" from a low power standby mode in which the power supplied to a utilization circuit is sharply reduced during time intervals between full power operations of the utilization circuit. The invention relates more particularly to such circuitry that allows use of external bypass capacitors to reduce noise signals.
Various high precision circuits such as 16 bit ADCs, DACs (digital-to-analog converters) and the like include reference voltage generator circuits. Some voltage generator circuits include a so-called "band gap" circuit. A band gap circuit produces a reference voltage that is proportional to absolute temperature. Often the output of the band gap circuit is coupled to an input of a buffer circuit. The buffer circuit typically has a very low output impedance, and produces a precise output reference voltage equal to the output voltage produced by the band gap circuit. The output reference voltage can be precisely adjusted during manufacture by trimming nichrome resistors or the like to produce the desired internal reference voltage level. For example, the assignee's ADS7800 12 bit analog-to-digital converter includes such band gap and buffer circuitry. The band gap reference voltage generating circuit produces high frequency thermal noise and flicker noise. Also, noise from other sources can appear on the output of the band gap circuit. Such noise typically has frequencies as high as 10 megahertz or higher. Magnitudes of such noise voltages are sufficiently large to cause inaccuracies in the least significant bits of the 12 bit ADC.
There are applications for high resolution ADCs, DACs, etc. in which very low average power consumption is a requirement. In some cases, low average power consumption can be achieved in a high resolution ADC or DAC by switching at least some of the circuitry into a low power consumption "standby" mode between conversions. However, this approach is impractical if large external bypass capacitors must be charged up every time the circuitry is switched from the low power standby mode into a "powered up" condition. Some prior circuits provide an external lead at the output of a buffer circuit having its input connected to the output of the band gap circuit, so that a user needing to filter the flicker noise and thermal noise generated by the band gap circuit can connect an external bypass capacitor of 1 to 10 microfarads to the output of the buffer circuit to low pass filter the voltage on the output of the buffer circuit. Low average power consumption is thereby achieved, but rapid powering up of the circuit is impractical.
There is an unmet need for a precise reference voltage generating circuit having the capabilities of both fast power-up from a low power standby condition and substantial reduction of high frequency noise generated within the reference voltage circuit.